A Typical Weakly Forced Mountain‐To‐Plain Extreme Precipitation Event Exacerbated by Urbanization in Beijing

Abstract

AbstractThe rainstorms that initiate over mountains, propagate to, and suddenly intensify in nearby plains pose great challenges for weather forecasters and the public. Being in a mountain‐plain transition zone, the city of Beijing is prone to rainfall extremes induced by weakly forced mountain‐to‐plain storms. Despite efforts to understand their possible mechanisms, it remains unclear whether and how these mountain‐to‐plain precipitation extremes are affected by urbanization in Beijing. Here we use a high‐resolution weather prediction model to hindcast a weakly forced mountain‐to‐plain extreme rainfall event impacting Beijing on 23 June 2011. We incorporate spatially varying urban canopy parameters into the model to assess their effect on rainfall simulation and find no significant improvement in model performance. By comparing simulations under current and pre‐urbanization scenarios, we find that urbanization intensifies the regional average rainfall within Beijing's Sixth Ring Road by ∼23%, with an increase in peak hourly rainfall rate reaching 27 mm hr−1. The higher surface roughness and enhanced vertical motions in urban areas facilitate the transport of warm and humid air from lower to mid‐upper layers, fueling the development and maintenance of convective storms. The dragging effect of urban canopy leads to more moisture accumulation and slower‐moving convections, resulting in more rainfall falling in urban areas. Our study underscores the key role of local urbanization in shaping the mountain‐to‐plain rainstorms in Beijing, which may help improve the forecast capacity for these weather extremes and related urban resilience planning.